1. Field of the Invention
The invention relates generally to the reception of uncompressed video over a wireless link. More specifically, the invention relates to the generation of a synchronization clock that is correlated to video frames received over a wireless link.
2. Description of the Prior Art
In common video reception systems, there are video control signals, such as the horizontal synchronization (Hsync) signal and the vertical synchronization (Vsync) signal. The Hsync and the Vsync signals may be used to generate a video pixel clock. Such signals may also be sent over the air for wireless transmission of video. However, the amount of bandwidth that is required to send these signals decreases the amount of bandwidth that is available for sending video data, i.e. significant portions of the available bandwidth are used to send such control signals.
The video source is usually comprised of a video data signal, such as RGB or YcCrCb, and controls, such as Hsync, plus Vsync or synchronization on green (SOG). Hsync and SOG provide information about the start of each video line. Vsync provides information about each video frame. When transmitting a video signal to a digital interface, the Hsync or SOG generate a video clock using a phase-locked loop (PLL). The PLL adjusts the clock such that it has a specific number of clocks between Hsync signals, although the Hsync itself may have a jitter. When Hsync jitters, the video source rate also fluctuates. In wired systems, generation of a video clock using Hsync allows the video system to accommodate the jittering of the video source.
Wireless systems traditionally use compressed formats as the video sources, and use a local oscillator for the video clock. Decompression of the video data is performed at the local oscillator frequency, using a local compressed buffer to accommodate clock rate variation. The extraction rate of the compressed video is achieved by using the local oscillator and, hence, the number of clocks in each line between Hsync signals may be handled using the local video clock oscillator.
In cases where the bandwidth is limited or better used for other purposes, for example, for the purpose of the transmission of video information and, in particular, in the case of the transmission of high-definition video information, the allocation of such bandwidth for these purposes is undesirable. For example, in a system described in U.S. provisional patent application Ser. No. 60/729,459, entitled Apparatus and Method for Uncompressed, Wireless Transmission of Video, assigned to common assignee and hereby incorporated by reference for all that it contains (hereinafter the '459 application), there is described such a system. Essentially, in the '459 system the control signals are not transmitted to the receiver and there is no timing information, other than the possibility of extracting a start of frame (SOF) indication.
In wireless systems that do not have any frame buffering or have a minimal buffering, for example the apparatus described in the '459 application, the prior art solutions are not applicable. Firstly, because the system is wireless the Hsync of the transmitter side is physically separated from the receiver side that generates the video clock output. Secondly, because there is no frame buffering a local oscillator cannot be used. The video source on the transmit side jitters relative to the local oscillator of the receive side and no buffering is available to accommodate for such jitter. Hence, in a wireless system having no frame buffering there is a challenge in generating the digital video output in a way that ensures that there are a predefined number of clocks between Hsync signals that are aligned with the video source.
It would therefore be advantageous to provide an apparatus and method thereof for overcoming the limitation of prior art solutions and that enables the generation of the desired clock over a wireless link. It would be further advantageous if such clock synchronization is of particular use with respect to the reception of a high-definition video transmission.